In leading to your partner's declared suit, always lead your highest card; this will enable him to tell what high cards are held against his suit and it will prevent your blocking his hand.
RETURN YOUR PARTNER'S LEAD
If your partner has had the original lead, RETURN HIS SUIT. There are very few "no-trump" hands where it is possible to bring in more than one suit, and if, instead of returning your partner's suit, you lead your own, you are playing for one suit and your partner for another, and as a result you will probably establish neither.
When it is evident that your suit is stronger than your partner's—i.e., if you have re-entry cards and can establish the suit in one lead—then, by all means, play for your own suit; but don't be deterred from returning your partner's lead simply because you see that the best card of his suit is against him. That card will have to make anyway, and by forcing it out of dummy at once you may enable partner to make the rest of his suit.
In returning your partner's lead, return your highest card. The importance of this is apparent: your partner can see the cards in his own and in the dummy hand, and if you return your best card he also knows what the dealer holds in that suit. It may prevent his leading up to the dealer's tenace; it may show him that the suit should be abandoned or that it should be again led from your hand. Returning the highest card minimises the risk of blocking the suit. Very often, by not getting rid of a 7, 8, 9, or 10 early in the hand, you make it impossible for your partner to make his small cards.
Don't be deceived by the dealer's play. His object is to fool you; and if he holds cards of equal value, he will probably take the trick with the highest.
Notice carefully your partner's first discard. It shows you the suit to lead and may also affect your own discard.
Don't, because the dealer leads the suit, refuse to take tricks with your aces and kings. By taking the trick, you may make a card good in your partner's hand. It is only the dealer who is in a position to know when to refuse tricks; he sees the two hands.
When there is no chance that your partner can take a trick in the suit led, it is sometimes wise to keep the commanding card until one hand cannot put the other in the lead, especially when there is no re-entry card in the hand with the long suit.
OPENING LEAD AT "NO-TRUMP"